1. Technical Field
This invention relates generally to a method for controlling engine transitions between lean and rich combustion modes, and more particularly to such a method for rapid air/fuel ratio control by which torque fluctuations and fuel penalties are minimized.
2. Background Art
Worldwide emissions regulations slated for introduction in the near future impose very stringent emissions regulations. The Tier 2 regulations in the United States require that Diesel vehicles have the same ultra-low emissions levels as spark ignited vehicles. Combustion mode changes, to address both in-cylinder (engine-out) and exhaust gas treatment device requirements have been proposed. For example, U.S. Pat. No. 5,732,554, issued Mar. 31, 1998 to Shizuo Sasaki, et al. for an EXHAUST GAS PURIFICATION DEVICE FOR AN INTERNAL COMBUSTION ENGINE describes a method by which the normal fuel lean operating mode of an engine is switched to a rich premixed charge compression ignition, more accurately and preferably referred to as a premixed controlled compression ignition (PCCI) combustion mode.
U.S. Pat. No. 5,937,639 granted Aug. 17, 1999 to Shizuo Sasaki, et al. for INTERNAL COMBUSTION ENGINE describes an alternative method for lowering the combustion temperature, i.e., low temperature combustion (LTC) to minimize smoke generation during rich, or near rich, combustion. LTC and PCCI combustion are alternative rich combustion modes which normal Diesel lean combustion can be transitioned to during engine operation.
Perhaps of most concern to the Diesel engine market are the proposed very tight future reductions in terms of oxides of nitrogen (NOx) and particulate matter (PM) emissions. One of the most promising technologies for NOx treatment is a NOx absorber, also known as a “lean NOx trap” (LNT). Diesel particulate filters (DPF) are the most likely, at least in the foreseeable future, means by which PM emissions can be reduced. Lean NOx traps and Diesel particulate filters need to be regenerated periodically to restore their efficiencies. The regeneration is usually done by providing reductants, such as CO and HC under oxygen-free conditions. This is typically provided by products of rich combustion within a defined temperature range.
Rich operation, i.e., a combustion mixture richer than stoichiometric, can be provided by throttling the intake air, increasing the exhaust gas recirculation (EGR) rate and varying turbocharger operation, all of which require additional fuel to provide rich operation. There are various methods for injecting the additional fuel necessary to provide rich operation and control intake throttle, exhaust gas recirculation, and turbocharger operation.
The present invention is directed to a method for fast air/fuel (A/F) ratio transitions, either from lean to rich or rich to lean, while maintaining constant torque. Also, to minimize fuel penalties, fast transition between standard Diesel lean combustion and rich combustion is desired. Heretofore, fuel used during such transitions was not useful for either NOx reduction, LTC rich combustion, or PCCI rich combustion because the required A/F ratio was not provided during the transition. The present invention is directed to overcoming the problems set forth above.